Generic storage catalysts are used for cleaning exhaust gases from an internal combustion engine (diesel engine), the storage catalysts removing nitrogen oxides (NOx) from the exhaust gas stream by storage. Sulfur oxides (SOx) are also removed in this manner. The loading of a storage catalyst with nitrogen oxides usually occurs over a period of up to approximately two minutes (depending on the operating point, but also for only approximately 0.3 to 1.0 minutes). The loaded catalyst must subsequently be regenerated. In the regeneration phase, nitrogen oxides are reduced to nitrogen and supplied to the exhaust gas. A reducing environment (rich mixture) with a prevailing air ratio λ<1 is necessary for this removal or regeneration process. The duration of these rich phases is approximately 1 to 30 seconds.
Rich phases may be produced either inside the engine by suitable combustion operations, or downstream from the engine by adding a reducing agent (diesel fuel, for example) to the exhaust gas. The addition of reducing agent downstream from the engine has the advantage that it may be performed independently of the operating point of the engine, unnoticed by the driver. It is possible to meet requirements such as constant engine response, constant torque, constant noise, and unchanged vehicle dynamics by the addition of reducing agent downstream from the engine.
In addition to the periodic NOx regeneration of the storage catalysts, these storage catalysts must also be desulfated but at longer time intervals. The desulfation generally occurs using a rich exhaust gas mixture (λ<1) at an exhaust gas temperature of 600° C. Periodic fluctuations about λ=1 are typically imposed here to avoid HC or CO breakthroughs. The duration of the desulfation is approximately 5 minutes. The desired conditions for this desulfation are difficult to establish inside the engine for the referenced duration, so that the opportunity again presents itself for adding reducing agent downstream from the engine.
To this end, the reducing agent is oxidized on an oxidation catalyst by the oxygen contained in the exhaust gas, with the release of heat. However, if the air ratio in the diesel exhaust gas is to be adjusted in this manner from approximately λ=2 to λ=0.95, temperatures above 1000° C. may arise, which would result in destruction of the storage catalyst.
U.S. Pat. No. 6,199,373 describes a method of desulfurizing an NOx storage catalyst by burning off at temperatures of at least 675° C. A modulation of the air ratio at a specified frequency and amplitude is proposed to this end so that the temperature is increased to the necessary level in the storage catalyst situated downstream from a three-way catalytic converter. O2, HC, and CO are essentially completely catalytically reacted exothermically in the storage catalyst, resulting in the desired temperature increase.
A similar method of desulfation is described in U.S. Pat. No. 5,974,788, in which the amplitude of the air ratio of the mixture supplied to the engine is modulated in such a way that oxygen is taken into the storage catalyst during a lean cylinder phase, and during a rich cylinder phase the oxygen then initiates the exothermic reaction required for the temperature increase.
A method of desulfating an NOx storage catalyst is described in U.S. Pat. No. 5,758,493 in which a portion of the engine cylinders operates using a rich mixture, and the other portion operates using a lean mixture, during the sulfation. The two exhaust gas streams are fed separately to the storage catalyst in order to undergo a catalytic exothermic chemical reaction in the catalyst. The resulting temperature increase is therefore sufficient to remove SOx.
A method based on the same principles, in which additionally any temperature drop in the storage catalyst is monitored and prevented, is known from U.S. Pat. No. 6,189,316.
Lastly, a method is known from U.S. Pat. No. 5,979,159 for desulfating a storage catalyst situated downstream from a three-way catalytic converter, in which an exothermic reaction is initiated in the three-way catalytic converter, producing an exhaust gas stream at elevated temperature which is used for desulfating the storage catalyst situated downstream.
In contrast to these known desulfation methods, the problem at hand is to limit the exhaust gas temperature which arises during the rich phase regeneration downstream from the engine to values which may be considered safe for the downstream components of the exhaust gas cleaning system, in particular for the storage catalyst itself, and which are sufficient for complete regeneration of the storage catalyst.